Memory cards are commonly used to store digital data for use with various products (e.g., electronics products). Examples of memory cards are flash cards that use Flash type or EEPROM type memory cells (also referred to as FLASH memory cells) to store the data. Flash cards have a relatively small form factor and have been used to store digital data for products such as cameras, hand-held computers, set-top boxes, hand-held or other small audio players/recorders (e.g., MP3 devices), and medical monitors. A major supplier of flash cards is SanDisk Corporation of Sunnyvale, Calif. Unlike standard MOS transistors, FLASH memory cells contain a floating gate that is electrically isolated and located between a control gate and a typically p-type substrate. Programming, i.e. injecting electrons into the floating gate, is generally accomplished by means of three major mechanisms:
Fowler-Nordheim (FN) tunneling, source side injection (SSI) and channel hot-electron injection (CHEI). During programming with FN tunneling, a high positive bias, called the program voltage (Vpgm), is applied to the control gate of a selected memory cell by biasing the appropriate word line while the associated bit line is held at about 0V whereas unselected bit lines are pre-charged to a high voltage, Vdd. Electrons tunnel from the substrate through the thin gate oxide and are then collected in the floating gate where they become trapped by the surrounding dielectric. In this way, the floating gate acquires a negative charge thereby raising the threshold voltage to a positive value and programming the cell to logic 0. Other operations that require high voltages include erase and program verify operations.
In order to provide the requisite high voltages for these operations (i.e., programming, erase, verify, etc.) a number of high voltage charge pumps provide the requisite voltages. However, with currently available high voltage charge pump designs, efficiencies are typically low, in the range of approximately 5% to 10% which means that the charge pump itself must use, for example, 10 mA of current in order to deliver 100 uA of current to an output node. In order to provide the requisite output voltage when connected to a load, a voltage regulator circuit coupled to the charge pump provides a constant DC current adding to the already relatively high current demand for conventional charge pump arrangements. This high current demand adds substantially to the overall power requirements of the memory device. For example, during program verify charge pumps used for programming are in regulation and burn power even though their respective outputs are not connected to their associated loads whereas during programming, the charge pumps used to program are in constant regulation after the desired voltage levels have been achieved. In any case, these additional current requirements unnecessarily add to the overall power consumption of the device and are therefore wasteful and costly.
Therefore, what is desired is a method, system, and apparatus for managing current used by high voltage charge pumps and associated regulators in a FLASH memory device.